Sometimes if you really look closely over your yard you can detect a slight depression where the original trench was dug. Also, typically the sewer line enters the house on the street side (you do have public sewage I assume?) and typically you should be able to figure out where and how deep it is by where your main stack goes through your basement wall. Keep in mind these things can be down 6 to 8 feet maybe you did'nt dig far enough. Good Luck